This application claims priority to Korean Patent Application No. 2005-71139, filed on Aug. 3, 2005 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to analog-to-digital conversion in CMOS (complementary metal oxide semiconductor) image sensors, and more particularly, to compensating for power noise during such analog-to-digital conversion.
2. Description of the Related Art
An Image sensor converts an image into electrical signals using photo-conversion. An image sensor is generally classified into either a charge-coupled device (CCD) type or a complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) type depending on the mechanism for generating and transferring charge carriers.
The CCD image sensor transfers electrons generated from incident light to an output port using gate pulses, and such transferred electrons are then transformed into a voltage. The CCD image sensor has relatively high optical sensitivity with reduced noise because an optical current is extracted after an accumulation time. However, the CCD image sensor disadvantageously has a complicated driving mechanism for continuous transfer of charge carriers. Additionally, the CCD image sensor consumes relatively high power.
The CMOS image sensor transforms electrons generated by incident light into a voltage in a pixel circuit. Such a voltage is output through a plurality of CMOS switches. The CMOS image sensor is generally inferior to the CCD image sensor in electro-optical characteristics. However, the CMOS image sensor has lower power consumption and higher integration density than the CCD image sensor.
In the CMOS image sensor, charge carriers are accumulated for a relatively short period of time before being converted into a voltage within each pixel circuit. Thus, such a voltage is more sensitive to noise such as from power voltage sources as the voltage signal is generated and transmitted.
An example CMOS image sensor includes an analog-to-digital (ADC) converter with single-ended inverting amplifiers instead of a differential comparator, for minimizing power consumption. Such a CMOS image sensor is especially amenable for use in portable electronic devices. However, the single-ended inverting amplifier has a trip point voltage that may be prone to power noise. Such noise for the trip point voltage causes image distortion. Thus, a CMOS image sensor with single-ended amplifiers having minimized image distortion from such power noise is desired.